Thermal processing involves a series of procedures by which an item may be exposed to a temperature-controlled environment, and is used in a variety of manufacturing procedures such as heat treating, quenching and refrigerated storage. One example of a thermal processor is a reflow oven. The production of various goods such as electronic circuit boards in solder reflow ovens frequently entails carefully controlled exposure to heating and/or cooling for specific periods. The elevated temperature conditions needed to solder component leads onto printed circuit boards must be gradually and uniformly applied to minimize thermal expansion stresses.
For this reason, convection heat transfer may be employed in these solder "reflow" operations. The connecting solder paste incorporates an amalgam of substances that must undergo phase changes at separate temperature levels. Solder reflow may be performed by sequentially passing a part (such as a printed circuit board to become a processed product) through a series of thermally isolated adjacent regions or "zones" in the reflow oven, the temperature of each being independently controlled. The part may be placed on a conveyor, which moves the part into the reflow oven entrance, through the zones, and out of the oven through the exit. The exposure of the part to the reflow oven conditions can be called a thermal process.
The thermal process may be controlled by establishing a control setpoint temperature T.sub.c for each zone and a conveyor speed u at which the part passes through the reflow oven. Convection being an empirically characterized mode of heat transfer, the task of conforming a part's temperature response to a target profile by adjusting setpoint temperatures and conveyor speed has traditionally used methods that require detailed information on the reflow oven physical dimensions and conveyor speed. This has included total distance of the reflow oven, the distance from the entrance to the start and end of each zone, as well as the precise time a part has entered and exited the reflow oven.
The setpoint temperatures for each zone in the reflow oven and the process profile for the processor temperature (serving as an algebraic simulation for each corresponding setpoint) may be described as functions of the relative zone lengths (e.g., fractions of the total distance of the reflow oven) or other independent variable associated with distance. These boundary condition temperatures are expected to have reached steady-state prior to the thermal process and hence not vary with time. By contrast, if a part temperature is measured, the thermal data may be recorded with respect to time, during the period that the part is conveyed through the reflow oven.
A prediction algorithm for correcting setpoint temperature (for achieving a target temperature profile for the part) may require a correlation between the processor temperature that varies in space, and a part temperature that varies in time. Such a correlation may depend on accurate measurements for distance between zones within the reflow oven, conveyor speed, clock cycle time, and synchronization references, such as an event time for a part to enter or exit the reflow oven. Such information may be subject to measurement uncertainty, leading to errors in adjusting part temperature response. Hence, a method to obviate such information is desired.